The present invention relates generally to a biasing device used with a two-way clutch assembly and, more specifically, to an apparatus for providing a biasing assembly for use with a two-way clutch assembly within a vehicle transfer case, which decreases the mechanical backlash that occurs during a counter-rotational engagement within the two-way clutch assembly.
Transfer cases are used in full and part-time, four-wheel drive vehicles to distribute driving power received through an input shaft from the vehicle""s transmission to a pair of output drive shafts. One of the drive shafts powers the vehicle""s front wheels and the other of the drive shafts powers the vehicle""s rear wheels. In vehicles that permit shifting between two-wheel drive (hereafter 2WD) and four-wheel drive (hereafter 4WD) modes, the input shaft of the transfer case provides continuous drive power to one of its output shafts and selectively provides drive power to the other output shaft via some type of disengageable or otherwise adjustable coupling, such as a viscous coupling, electromagnetic clutch, or positionable spur gearing. Other drive modes are sometimes provided, including 4WD high (4H) for higher 4WD speeds, 4WD low (4L) for lower 4WD speeds, neutral for disengaging the transmission from the front and rear axles to allow towing, and locked 4WD for controlling wheel slippage.
Additionally, other transfer case applications have evolved, such as xe2x80x9con-demand 4WDxe2x80x9d, in which a transfer case, with its related parts that provide 4WD, is installed in the vehicle, yet 4WD mode is only engaged, by automatic means, when there is a loss of 2WD traction. Full-time, or constant, 4WD mode, sometimes referred to as xe2x80x9call-wheel drivexe2x80x9d is also currently available in some automotive variants. In this mode, 4WD is not deselectable and remains a fixed configuration.
In the transfer cases used for these vehicles, certain elements, or components, are required to transmit the driving force. More particularly, certain elements are required to selectively transmit the driving force during particular driving situations and not in others. One example of a device used to selectively transmit driving, or rotational force, in a transfer case is a one-way clutch. One-way clutches are known devices having inner and outer races with an engagement mechanism interposed therebetween. Generally speaking, the engagement mechanism is designed to lock the races together when the relative rotation of the races is in one particular rotational direction. When the races rotate in the opposite relative direction, the engagement mechanism is unlocked and the races have free rotation relative to each other. In application, when the races are fixed to concentric shafts, the one-way clutch will function to hold the shafts together when engaged, causing them to rotate in the same direction and thereby transferring motive force, or drive torque, from one shaft to the other. When the one-way clutch is disengaged, the shafts thereby freewheel relative to each other.
Specific applications govern how the one-way clutch engagement is designed. A one-way clutch may be designed to have one race as the driving member and one as the driven member, or the clutch may be designed to allow either shaft to act as the driving member during different operating modes. In this manner, the locking mechanism of the one-way clutch may be designed to engage in response to the rotation of only one of the races or the clutch may be designed as to engage if either race provides the proper relative rotation.
The one-way clutch is typically used in circumstances in which shaft to shaft, or shaft to race, rotational, torque-transferring engagements are desirable but a xe2x80x9chardxe2x80x99 connection, such as a spline or keyed connection, would not work. For example, during certain operating parameters, a 4WD vehicle experiences driveline difficulties that arise from having the front and rear wheels cooperatively driven, which can be alleviated by the use of these one-way clutch devices within the transfer case. When a 4WD vehicle turns a tight corner with the four wheels coupled together on a paved road, the vehicle may experience what is known as xe2x80x9ctight corner braking effectxe2x80x9d. This happens due to the inherent physical geometry that affects objects rotating at different radial distances from a center point. Two distinct effects generally occur with 4WD vehicles. First, when any vehicle enters a curve, the wheels on the outside of the curve must traverse a greater circumferential distance than the wheels on the inside of the curve due to the greater radial distance from the center of the curve. The tighter the curve, the greater the difference in the rate of rotational, angular speed between the inner wheels and the outer wheels. Therefore, in a curve the outside wheels must rotate faster than the inner wheels. This is effect is exaggerated in a 4WD vehicle but is generally countered by the vehicle""s differential assemblies installed at the front and rear axles. Secondly, since the front wheels are also leading the vehicle through the curve, they must rotate faster than the rear wheels. With a solid 4WD engagement there is no device (such as a differential) to counter this action and the slower moving rear wheels act in an undesirable braking manner.
To solve this problem, one-way clutches have been employed in the transfer case so that as the vehicle beings turning a corner, the front wheels (connected to the transfer case output shaft through a one-way clutch) are allowed to disengage and freewheel faster than the rear wheels. Specifically, the driven shaft of the one-way clutch (i.e., the output shaft to the 4WD front wheels) begins turning faster than the input, or driving, shaft and the one-way clutch""s locking mechanism disengages allowing freewheeling of the output shaft relative to the input shaft. This momentarily takes the transfer case out of 4WD and prevents the tight corner braking effect.
Another undesirable 4WD driving effect happens during engine braking. This occurs in a manual transmission 4WD vehicle when in 4WD and coasting. The manual transmission maintains the physical connection to the vehicle""s engine, such that when the vehicle is allowed to coast, the engine places a decelerating, or braking, force on the transfer case, both the input shaft and output shafts, and ultimately on both the front and rear wheels. The normal, and undesirable, parasitic effect of engine braking though the rear wheels of a manual transmission 2WD vehicle has a negative impact on fuel consumption and efficiency, which is greatly increased in the case of the 4WD vehicle by adding in the front wheels as well. In this instance, when a one-way clutch is used in the driveline of the transfer case, the slowing of the input shaft through the engine braking effect allows the output shaft (which is connected to the front wheels) to disengage and freewheel, momentarily taking the transfer case out of 4WD and preventing the engine braking effect from passing to the front wheels, thereby reducing the negative impact on fuel efficiency.
Finally, in an on-demand application, a one-way clutch can be employed in the transfer case so that in the normal 2WD mode, if one of the rear wheels should slip during vehicle acceleration, the rotating speed of the input shaft will increase, so that the one-way clutch engaging elements will bring the transfer case into 4WD and the front wheels into a driven mode.
While proving to be of great value, as transfer case design technology utilizing one-way clutches continued to evolve, the one-way clutch designs began to reveal certain limitations. Most importantly, while a one-way clutch would solve the above-mentioned problems and disadvantages, the one-way clutch would only work, by itself, in one direction. In other words, the one-way rotational engagement between the input and output shafts in the transfer case would allow forward 4WD movement but not reverse 4WD movement. To provide this function, additional mechanisms and devices were added to the transfer case to supplement the one-way clutches. However, this added weight and complexity to the transfer case.
The concurrent on-going design goals of reducing the mechanical complexity and physical bulk of transfer cases while increasing their functionally brought about the design of another torque transmitting device that adapted the one-way clutch mechanism to allow engagement in a bi-rotational or two-way manner. This device is typically known as a two-way clutch. The two-way clutch is desirable to solve all the above difficulties with 4WD and provide full forward and reverse functionality. It allows the input shaft to be designed as the driving member for 4WD modes, in both rotational directions, but offers bi-directional freewheel movement of the driven output shaft as needed when the input shaft is stationary or rotating slower than the output shaft.
Yet, even though the conventional two-way clutch design has been very useful in solving these and other 4WD driving difficulties, it has become apparent in applications that use a two-way clutch for 4WD engagement that certain deficiencies still exist which cause particular problems. Specifically, there exists a physical angular distance from the engaged interconnection between the races of the two-way clutch from a first rotational direction to the engagement of the races in the reverse, or second direction. This angular distance, also known as backlash, can cause mechanical problems as the two-way clutch is repeatedly called on to change its driving rotational direction over the service life of the transfer case. This is due to the mechanical load brought to bear in the switch from one rotational direction to the other. This rotational shift takes the form of a high-impact shock loading that is not only absorbed by the two-way clutch but is also translated to the other components attached to the two-way clutch in the driveline, all to a repetitive detrimental effect. This shock loading is detrimental as it reduces component life and reliability, while adding unpleasant ride characteristics to the vehicle.
Some attempts have been made to reduce the amount of backlash within a two-way clutch assembly but these have generally required substantial, or radical, redesigns of the transfer case structure. In the typical two-way clutch currently used, the structurally inherent backlash can only be physically reduced to between 4 and 5 degrees of rotation. Even this seemingly small amount of backlash causes the problems mentioned above.
Therefore, there exists a need to create an improved, two-way clutch assembly for use as a driveline component within a transfer case that has a reduced, or minimal, backlash, which will thereby reduce impact loading, extend the life of the clutch and associated components, and improve the ride characteristics of the vehicle.
The present invention overcomes these deficiencies in the related art in a biasing device used with a two-way clutch assembly and, more specifically, in an apparatus for providing a biasing assembly for use with a two-way clutch assembly within the case of a vehicle transfer case that decreases the mechanical backlash that occurs during a counter-rotational engagement within the two-way clutch assembly.
Specifically, the present invention provides a two-way clutch biasing assembly that includes a two-way clutch having an inner race operatively attached to an input shaft and an outer race operatively attached to an output shaft. An engagement assembly is operatively interposed between the inner and outer races and is adapted to rotate bi-directionally thereby providing mechanical engagement between the races. The present invention also includes a planetary gear assembly having a ring gear, a plurality of planetary gears operatively connected to a carrier, and a sun gear that is operatively connected to the engagement assembly. The planetary gear assembly is adapted to operatively rotate the engagement assembly within the two-way clutch. Also, a drag clutch assembly is operatively connected to the carrier to provide a mechanical force to hold the carrier stationary when the input shaft is stationary, such that the sun gear is rotated by the plurality of planetary gears, thereby rotating the engagement assembly between the races of the two-way clutch and mechanically engaging the races together when the input shaft begins to move.
The present invention thereby overcomes the disadvantages and drawbacks of the current art by decreasing the mechanical backlash found in conventional devices by providing an assembly that has a mechanical advantage, which acts as a speed multiplier, allowing the two-way clutch engagement mechanism to traverse the angular displacement between the two engagement regions of the two-way clutch assembly at a rate several times faster than would possible by the mechanism itself.